Turkey does not differentiate between what it sees as terrorist organisations and any support for Syrian Kurdish forces means supporting the Kurdistan Workers’ Party (PKK), state-run Anadolu news agency quoted presidential spokesman İbrahim Kalın as saying on Wednesday in reference to U.S. backing for Kurdish fighters in Syria battling Islamic State.

Turkey sees the People’s Protection Units (YPG) and its political wing, the Democratic Union Party (PYD), that control most of northeast Syria as part of the PKK that has been fighting for self-rule inside Turkey for more than 30 years. The YPG denies being part of the PKK, but both groups share the same democratic confederalist ideology and are part of the same umbrella organisation.

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While the United States recognises the PKK as a terrorist organisation, U.S. troops have armed and trained YPG fighters who make up the bulk of the U.S.-backed Syrian Democratic Forces (SDF) that has pushed Islamic State (ISIS) out of most of northern Syria.

“Our position against the PKK is clear. Contrary to the PKK, we do not define the YPG as a terrorist organisation. We have never done that,” Washington's Syria envoy James Jeffrey said .

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Turkey last week announced that it would launch a large-scale operation against the YPG east of River Euphrates in Syria and began shelling YPG positions. As a result of the Turkish shelling, the SDF called a temporary halt to its offensive against the remnants of ISIS in Syria. Jeffrey said on Wednesday that eliminating ISIS in Syria was Washington’s top priority.

“Unilateral military strikes into northwest Syria by any party, particularly as American personnel may be present or in the vicinity, are of great concern to us,” State Department spokesman Robert Palladino said last week in response to the Turkish shelling of Kurdish-controlled areas.